The present invention relates generally to detecting an attenuation of secondary cosmic radiation passing through a given depth of soil as a means for determining the water content in the soil.
In many agriculture regions of the country knowledge of the water content in the surrounding soils is absolutely critical to the viability of agricultural enterprises in these areas. In particular, wine growing regions in the drier valleys of California may receive about 15 inches of precipitation a year. All of that total, however, comes as rain between the months of November through March after which virtually no rain falls for the remainder of the year. Irrigation is therefore essential to the sustainability of this crop. The cost of water, however, makes it just as essential to limit the amount of water applied to only that which is necessary for the health of the plant. Consequently, there has been a long felt need for a simple, inexpensive, reliable and practical method for determining these data. For these same reasons, any country whose agribusiness includes a segment which is water intensive or which utilizes arid and drought susceptible acreage would be interested in soil hydrology.
As the demand on water resources increases there is a corresponding need to automate more survey sites in order to measure more accurately the content of moisture in the soil in order to allocate resources appropriately: reliance upon core samples is too expensive and time consuming to provide much more than a periodic spot survey of water usage. As a result, a number of sensing technologies have been developed to provide this information. These are conveniently divided into five categories based on the physical principles on which each is derived. In particular, these are hygrometric, tensiometric, gravimetric, electromagnetic, and nuclear techniques and a useful summary of each of these is available in Engineer Technical Letter (ETL) No. 1110-2-323 (dated Jun. 24, 1991) by the U.S. Army Corps of Engineers.
Most of these techniques, however, provide soil moisture information that is limited to regions at or near the surface of the soil. What is needed instead is a system to provide subsurface measurements of the amount of water held in the intervening layer of soil. The exception to the techniques described above are various active and passive radioactive techniques for measuring the water content in soils and in snowpacks that have been reported in the literature. In particular, Abelentsev et al., in U.S. Pat. No. 4,992,667, disclose a method for simultaneous measurement of moisture in soils and in snowpacks utilizing fixed neutron detectors to monitor the attenuation in extraterrestrial or xe2x80x9ccosmic rayxe2x80x9d generated neutron radiation versus snow depth. In addition, the Applicant, in U.S. Patent Ser. No. 5,594,250, discloses using the attenuation of secondary cosmic gamma radiation by water molecules to determine the equivalent water content in a snowpack. Neither of these patents, however, disclose or suggest that it is possible to provide a measure of water suspended in a layer of soil above a detector by measuring the attenuation of secondary cosmic rays.
The present invention discloses method and a system for determining the water contained in a layer of soil across a cultivated field which, by measuring the attenuation of a wide spectrum of secondary cosmic radiation, has significant advantages over existing techniques.
The invention disclosed herein provides a new solution to the problem of measuring soil moisture at depths of more than a few feet, particularly in remote and widely dispersed locations. A method is described which permits a relative measurement of soil moisture using an apparatus which is practical and inexpensive, easy to install, requires little or no maintenance, and makes available data in xe2x80x9creal timexe2x80x9d. The present invention measures the attenuation of secondary cosmic radiation, primarily gamma rays and muons, rather than extraterrestrial neutron radiation, or other active, terrestrial radiation sources to determine the moisture content of soils at varying depths below a cultivated field.
Cosmic radiation is a stream of ionizing radiation of extraterrestrial origin that enter the Earth""s atmosphere consisting chiefly of protons, alpha particles, diminishing proportions of most other atomic nuclei of increasing mass through iron, and high-energy electrons. These high energy particles eventually collide with the atomic nuclei of air molecules in the upper atmosphere and generate a cascade of secondary radiation known as secondary cosmic radiation, as a consequence. These high energy collisions result in the production of neutral, charged pi mesons as well as high energy protons and neutrons (nucleons). The neutrally charged pi mesons decay almost immediately into gamma rays which in turn decay into electron-positron pairs which themselves lead to generations of gamma/electron-positron decay reactions, while the positively and negatively charged pi mesons decay into mu mesons and neutrinos, of which about 5% of the mu mesons, or xe2x80x9cmuons,xe2x80x9d reach ground level. The energetic nucleons (protons and neutrons) generate a cascade of successive nuclear decay reactions to produce successive generations of protons and neutrons having successively lower energies. Because of ionization losses as they progress through the atmosphere, however, the dominant form of nucleons reaching ground level are thermalized neutrons.
The overall result of these nuclear interactions is a shower of energetic particles at ground level made up principally of muons, neutrons, electrons and photons (gamma rays). In particular, the cosmic gamma radiation and muons can serve as a radiation source since it is known that for a given detector volume, the cosmic gamma ray flux is about 100 times greater than that for cosmic neutrons.
The invention described herein is directed to a method of remotely determining the relative content of water in a surrounding volume of soil at given soil depths. Accordingly, it is an object of this invention to use the attenuation of secondary cosmic radiation by water suspended in soil aggregates to determine the relative water content of those soils.
Another object of this invention is to perform this measurement remotely and transmit the data in real time.
This invention can be implemented with a detector such as a scintillator type radiation detector of the type used to provide a response to high energy particles and radiation. The scintillator and its associated instrumentation are intended to be located at several feet below the soil surface. As precipitation falls or as irrigation is applied water accumulates over time in the surrounding soil. The water content is determined by monitoring the decrease in secondary cosmic radiation intensity as the quantity of water in the soil increases. Where a scintillation type detector is used to detect secondary cosmic rays, a photodetection system and a pulse height analyzer are used to count the number of particles penetrating through the soil to reach the detector. A power supply coupled with a telemetry system for transmitting data from remote locations completes the instrument package. Because variations in the flux of cosmic radiation throughout the year can lead to errors in the determination of the water content of the soil, a second embodiment of this invention employs a second detector positioned above the surface of the soil to record these variations. The data received from the second detector may then be used to correct attenuation data. In another embodiment of this invention, a second scintillator of similar size can be placed directly below and contiguous with the primary detector to allow anti-coincident exclusion of false readings due to the high energy primary cosmic radiation. The inventor has determined that while secondary cosmic rays generally could be employed to perform the measurements described herein, there is a preferable energy range that is desired. The preferred energy range is from above about 5 MeV, and is high enough to measure soil moistures that could reasonably be expected to be encountered but low enough to make the detector size of practical dimensions.
The objects of the present invention together with additional objects, novel features and advantages thereof over existing prior art forms, which will become apparent to those skilled in the art from detailed disclosure of the present invention as set forth hereinbelow, are accomplished by the improvements herein described and claimed.